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The Fiat Croma is an innovative car with plenty of substance, which stands out both for the second generation Multijet engines and the lavish array of comfort and safety features. This is why the car is the new benchmark for its segment.
Diesel engines: the history of a Fiat success
Beneath very individual styling, the new Croma hides a comprehensive range of excellent engines, which embody advanced technologies and generous performance. The three Multijet turbodiesel engines deserve a separate mention and they are joined by two powerful, smooth petrol engines, all of which respect the strict Euro 4 emissions standards.
Taking the petrol engines first: the 1.8 16v (available after the launch) delivers a maximum of 103 kW (140 bhp) and peak torque of 175 Nm (17.8 kgm) at 3,800 rpm. The 2.2 delivers 108 kW (147 bhp) at 5,800 rpm and peak torque of 203 Nm (20.7 kgm) at 4,000 rpm. The top speed is 210 km/h and it accelerates from 0 to 100 km/h in 10.1 seconds (when the engine is combined with the sequential automatic transmission the values are 205 km/h and 10.7 seconds respectively). Excellent performance in other words, but consumption is low: 11.5 l/100 km in the urban cycle, 7 l/100 km in the out-of-town cycle and 8.6 l/100 km in the mixed cycle (with the automatic transmission the values are 13, 7.8, and 9.7 l/100 km). Both engines are available with 5-speed gearboxes (the sequential automatic transmission is also available with the 2.2 16v).
Now we can consider the three Multijet turbodiesel engines (a 1.9 delivering 120 or 150 bhp, and the new 200 bhp 2.4), all combined with 6-speed transmissions (including the automatic unit available on the 150 bhp 1.9), jewels of modern engineering which confirm Fiat Auto's leadership in this field. When it developed the Multijet system, the Fiat Group set an important new record in the field of diesel engines, made possible by the huge amount of expertise accumulated since 1986, when the Croma TDI was launched, the world's first direct injection diesel-engined car. For those years it was a brilliant result, the first important step forward towards more efficient combustion in the field of diesel engines for cars. Thanks to this technology, which other manufacturers soon adopted, diesel-engined cars were able to guarantee both better performance and lower consumption. One problem remained: the excessive noise made by the engine at low speeds and in "speed transients".
This is where the history of the Unijet starts, or rather the search for a more evolved direct injection system, which could drastically reduce the problem of noisy combustion. After a few years, this research produced the Unijet, but it also brought other advantages in terms of performance and consumption. There were only two possible ways of solving the problem: passive acceptance, isolating the engine to prevent the sound waves from spreading, or working actively to eliminate the problem at source, developing an injection system that could reduce combustion noise. Having chosen the second strategy, the Fiat engineers immediately concentrated their research on the "Common Rail" principle, excluding other high pressure injection systems after careful analysis. The other systems did not allow the pressure to be managed independently of the rpm and the engine load, nor did it envisage pre-injection, the two strengths of the Unijet.
The theory that the engineers started to analyse was both simple and ingenious, the fruit of work done by researchers at Zurich University, which had never been applied on a car. By continuing to push diesel fuel into a tank, pressure builds up inside the tank, which thus becomes a hydraulic accumulator, or "rail", in other words a reserve of pressurised fuel, ready for use. Pre-industrialisation of the Unijet, the system developed by Magneti Marelli, Centro Ricerche Fiat and Elasis on the basis of the Common Rail principle, began three years later, in 1990. This stage was concluded in 1994, when Fiat Auto decided to choose a partner with great experience in the field of injection systems for diesel engines. The project was therefore sold to Robert Bosch for the final part of the work, i.e. completion of the development process and industrialisation.
In October 1997, eleven years after the Croma TDI, another record-setting car went on the market: the Alfa 156 JTD equipped with a revolutionary turbodiesel engine that gave hitherto unimaginable results. The cars equipped with this engine were incredibly quiet, with a sparkling response on a par with that of a petrol engine and performance levels that were 12% higher than those of a similar pre-combustion engine, as well as 15% lower consumption. The success of the Alfa 156 with the JTD engine was immediate, and very soon not only was it being used on other Fiat Auto models, but numerous other car-makers were adopting similar engines. Now it is the turn of the second generation of JTD engines, the Multijet multivalve units.
The Multijet system, secret of second generation JTD engines
The underlying principles of second generation turbodiesel engines remain the same, i.e. the Common Rail, high injection pressure and electronic injector control. But one extra feature has been added: during each engine cycle, the number of injections increases over and above the current number of two. In this way, the same amount of diesel is burnt inside the cylinder but in several portions to achieve smoother combustion. The advantages include lower running noise, reduced emissions and a 6-7% increase in performance. All this comes with a level of engine efficiency that improves car handling still further.
These results are not to be underestimated, particularly if it is remembered that they are obtained with an engine that represents an incredible leap forward from prechamber diesels and even improves on first generation JTD engines.
The secret of the Multijet engine lies in the control unit that governs the electric injector opening and closure system (and also in the injectors themselves). The crucial element is the electronic control unit itself that can perform a set of injections that may be very closely spaced. Fiat Auto's researchers developed the part (together with the injectors) especially for this application. It is designed to deliver the multiple injections that assure more accurate control of pressures and temperatures developed inside the combustion chamber and also more efficient use of air taken into the cylinders. This enables further goals to be achieved: quieter combustion, reduced emissions and increased performance.
The Multijet system is underpinned by long years of research. Our engineers began by resolving the problem of the limits imposed by the control units. Then they went on to map the benefits they could achieve by plotting different multiple injection sequences (two secondary injections very close to the main injection; one secondary injection not too close to the main injection plus two closely-spaced secondary injections; one secondary injection and then two main injections close together after a certain period etc.) against different engine service conditions: in the idling region; with low loads and low rpm; with high rpm and moderate load; with low rpm and high load etc..
The study revealed the potential of the system and showed that great benefits are achievable in all cases, though these tend to focus on one field or another according to the type of sequence chosen and the engine service area targeted. In some cases, for example, the priority is to reduce start-up times and fume levels, in other cases it is to increase torque and reduce noise while in others it is to reduce emissions and ensure a quieter drive. And now this research strand has led to the creation of the Multijet engines, including the brand new 200 bhp 2.4 JTD Multijet 20v which debuts on the new Croma.
The 200 bhp 2.4 Multijet 20v
Derived from the familiar 2.4 JTD 10 valve "Common Rail" engine, the new 20-valve unit has 5 cylinders in line, with a bore of 82 mm and stroke of 90.4 mm. It is the most powerful engine in the JTD multivalve family with Multijet technology, with a specific power of 83 bhp/litre.
The new engine offers numerous advantages. First of all, during the heating stage, it is from 3 to 6 decibels quieter, depending on the rev speed and ambient temperature. It is also extremely powerful (147 kW - 200 bhp at 4000 rpm) with a generous torque (400 Nm - 40.8 kgm at 2000 rpm).
There are four valves per cylinder, activated via hydraulic tappets and rocker arms directly by the twin overhead camshaft. Several changes were made to the new turbodiesel engine to boost performance and engine torque at low speeds, and to eliminate noise and vibration. For example, the "Common Rail" system adopted on the 2.4 JTD Multijet 20v envisages two new automatic control strategies for the setting and balance of the injected diesel fuel, which reduces noise and vibration.
The engine includes a number of new components: the cylinder head with hydraulic tappets at the con rods and a steel propeller shaft, and the piston with a hollow core to carry the cooling oil to the journal and con rod bearings which are made of a new material. The exhaust and intake manifolds are also new: the former is of a special high yield material, while the latter is made of pressure die cast aluminium.
The electronic EGR exhaust gas cooling system has also been modified; the lubricating circuit has a new oil pump and an external air/oil heat exchanger to cool the oil, and the water pump on the cooling circuit is new. In other words, a long list of improvements and changes that have produced a reliable, powerful engine that is sparing on fuel. Excellent results, achieved thanks to different engine control settings, an increase in the direct injection pressure from 1350 to 1400 bar, and new turboblower settings.
The turboboost is provided by a KKK 2080 turboblower with a variable geometry turbo that helps to improve power delivery, but also generates very high torque at low engine speeds. In fact, 90% of peak torque is available between 1750 and 3500 rpm. These figures translate into extremely enjoyable driving and brilliant performance: the new Croma has a top speed of 216 km/h and accelerates from 0 to 100 km/h in 8.5 seconds. And fuel consumption remains low: 8 l/100 km in the mixed cycle, 11.5 in town, and 6 out-of-town. In addition to which the new 200 bhp 2.4 JTD Multijet 20v adopts a new 6-speed sequential automatic transmission, a compact light device that incorporates the control unit in the gearbox, which was designed paying particular attention to consumption. On one hand, the use of low friction oil has boosted efficiency, while on the other, the sixth speed has been calculated particularly for use on the motorway.
Finally, the 2.4 JTD Multijet 20v (like the other two Multijet engines in the range) respects Euro 4 legislation and offers a particulate trap (DPF), a "for life" system that eliminates fine dust and does not need additives to regenerate it.
The 150 bhp and 120 bhp 1.9 Multijet
Both have 4 cylinders in line, a bore of 82 mm and stroke of 90.4 mm; the first delivers 150 bhp (110 kW) at 4,000 rpm and peak torque of 320 Nm (32.6 kgm) at 2,000 rpm; the second delivers 120 bhp (88 kW) at 4,000 rpm and peak torque of 280 Nm (28.6 kgm) at 2,000 rpm. Changes were made to both engines to boost performance, engine torque at low speeds and to reduce noise and vibration. For example, the Common Rail system includes two new strategies for automatically calibrating and balancing the diesel injected to lower noise and reduce vibration. And both engines are equipped with variable geometry Garrett VGT 17 turbines. With the 150 bhp 1.9 Multijet engine, the Fiat Croma has a top speed of 210 km/h, and accelerates from 0 to 100 km/h in 9.6 seconds (the values for the 120 bhp 1.9 Multijet are 195 km/h and 11.3 seconds respectively). In spite of this brilliant performance, fuel consumption is limited: 8.2 l/100 km in the urban cycle, 4.9 l/100 km out-of-town and 6.1 l/100 km on the mixed cycle (7.9, 5 and 6.1 are the values for the 120 bhp 1.9 Multijet).
A pleasure to drive and travel in
The Fiat Croma surrounds the driver and passengers with technologies that ensure outstanding acoustic and climatic comfort. The result is a muted, enveloping environment, free from vibration thanks to the careful soundproofing of the car and the outstanding torsional and flexural rigidity of the bodyshell. The Fiat Croma also offers an automatic "dual-zone" climate system, a suspension system that makes the new model even easier to handle and "isolate" it from the road, electro-hydraulic power steering that simplifies every manoeuvre, a powerful braking system that is effective in all situations, tyres designed specifically for superb comfort and maximum safety, comfortable, functional seats, a huge panoramic opening sunroof, a modular luggage compartment and numerous storage compartments. In other words, the new model is unbeatable where comfort is concerned.
NVH comfort
The Fiat Croma has achieved a high standard of comfort which translates into quiet, vibration-free driving. The quietness of a car is expressed by the conversability index and this is an excellent 65% at 140 km/h on the new model. This result was made possible both by the usual package of sound-deadening elements and the introduction of no less than 18 expandable partitions (they are positioned inside the boxed elements where they prevent noise from propagating and isolate the passenger compartment from the outside world), and by an innovative windscreen. The 2 sheets of glass are separated by a film of PVB plastic, adopting multiply technology to enhance acoustic comfort. The different properties of the multiply material result in optimal damping of the sound waves: the noise inside the passenger compartment is attenuated, particularly in the 200 kHz frequency band (the wave frequency in which the most annoying noises occur on the road). Vibration is limited by a particularly rigid bodyshell structure. Flexural rigidity is excellent at 960 daN/mm, while torsional rigidity is 100,600 daNm/rad (outstanding on a car with a tailgate). Particular attention was also paid to the rigid attachment of all the mechanical organs: for example, this made it possible to obtain a vertical vibration frequency for the steering wheel above 38 Hz. This was enhanced by the large sections of the main boxed elements and the presence of a "torsion box" that connects the 2 wheelarches behind the rear seats.
The suspension on the Fiat Croma was designed to achieve the highest possible levels of comfort and handling, with MacPherson struts at the front and 4-link Multilink suspension at the rear, and comfort is excellent on all types of surface. Handling is precise and immediate, on a par with the best competition in this segment. The level of comfort is equally high on the front and rear seats, in all conditions. The front suspension system is supported by a frame which is anchored elastically to the chassis, to separate the incoming stress from the frequencies of the housing, and to improve the perceptible acoustic response in the car when the front wheels negotiate an obstacle. The rear suspension is supported by a subframe that is anchored rigidly to the structure to optimise the precision of the response of the rear end, guaranteeing immediacy and stability.
Comfort on the road is guaranteed by the outstanding flexibility and generous suspension excursion. The rear springs are of the variable flexibility type to maintain the frequency of the housing as constant as possible, and to adapt to the many possible load configurations. The damper system was designed to optimise driving comfort on surfaces with repeated rough patches, and a basically symmetrical setting was chosen. In order to limit rolling on the Fiat Croma, and to make the car more agile in spite of its size, the understeer of the centering was limited and neutralised, by adjusting the stabilising ratio of the front and rear bars.
Front suspension
The front suspension adopts a MacPherson layout and it is mounted on a hydroformed steel frame. The system is independent and the dampers act as both damping and bearing elements, working together with a transverse link to guide the hub during oscillation. The advantages of the MacPherson system are its small size and the possibility of adjusting the changes to the suspension trim under load. It also made it possible to achieve the best possible compromise between handling and comfort right at the design stage.
Rear suspension
The Fiat Croma incorporates a Multilink suspension system into the rear axle with four links (three transverse and one longitudinal) connected to a supporting frame to guarantee excellent dynamic comfort. Multilink suspension technology is the best available today and solves the problems related to longitudinal and transverse forces independently. For example, lateral forces are absorbed by the three transverse links. The length, layout and rigidity of the wheel supports were specially designed to maintain the wheels in an optimal position under load, thus influencing the behaviour of the car when cornering.
And thanks to its longitudinal and transverse links, the Multilink system allows extremely precise vertical movements of the wheels, while at the same time limiting longitudinal and transverse displacement; this makes for excellent roadholding. And finally, light alloys were used extensively to improve the suspension behaviour, in particular for the front and rear pillars, and the aluminium front transverse link.
Electro-hydraulic power steering (EHPS)
The power steering system on the Fiat Croma is of the electro-hydraulic type, and its intervention varies with the speed of the car and the steering lock. The power steering is programmed to decrease its effect as the speed increases. This means that the driver can turn the wheel very easily when parking, but maintain a safe contact with the road at high speed. In practice, the system varies the hydraulic power assist (number of pump revolutions) according to the steering rotation speed and the vehicle speed, providing more assistance for parking and very low hydraulic assistance at high speed. And because the system is only triggered during steering manoeuvres, it is also advantageous in terms of fuel consumption over a conventional hydraulic system that is always driven by the engine.
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